We are investigating certain aspects of nucleic acid metabolism in two strains of Entamoeba histolytica (viz., strains HK9 and Laredo) grown exenically in Diamond's TP-S-1 medium. Where feasible, similar studies will be done with the HK9 strain, grown monoxenically with Bacteroides sp. in a modification of Diamond's TP-S-1 medium vs monoxenic CLG medium. These studies, using the techniques of radioisotope precursor incorporation as detected by autoradography and liquid scintillation spectrometry, will examine: 1) relative rates and nuclear sites of synthesis of DNA and RNA; 2) anabolic pathways of nucleotide synthesis (i.e., de novo vs salvage metabolism) by following incorporation of radioisotopically labelled purine and pyrimidine precursors; 3) kinetics of incorporation of isotpically labelled free bases vs nucleosides into nucleotide pools and whether preferences exist for various salvage precursors; 4) the stability of "pulse labelled" vs "long-term" labelled RNA; and 5) the isolation and classification of "pulse" vs "long-term" labelled RNA species of axenically grown E. histolytica by employing established biochemical extraction procedures $ and gradient ultracentrifugation techniques.